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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #215414

Title: Sensible heat observations reveal soil-water evaporation dynamics

item Sauer, Thomas

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2007
Publication Date: 2/26/2008
Citation: Heitman, J., Horton, R., Sauer, T.J., Desutter, T. 2008. Sensible heat observations reveal soil-water evaporation dynamics. Journal of Hydrometeorology. 9:165-171.

Interpretive Summary: Accurate measurement of the rate and amount of evaporation of water from soil is important because the amount of water in the soil affects plant growth and many other processes that occur in the soil. Methods have been developed to measure soil water evaporation based on the amount of energy it takes to evaporate water or measurement of changes in soil water content. Attempts to connect these two approaches have never been successful under field conditions. This objective of this study was to use new, small-scale sensors near the soil surface to measure both soil thermal properties and soil water content at the same time. The sensors were placed in the soil very close to the surface and then monitored over time as the soil wetted and dried with rain and sunshine. Soil water evaporation was measured separatedly by weighing cores of soil for comparison. The new method succeeded in not only accurately measuring soil water evaporation but also in identifying which layers in the soil the water was evaporating from. This research is important for scientists interested in measuring soil water evaporation directly from small areas and especially from specific layers in the soil.

Technical Abstract: Soil water evaporation is important at scales ranging from microbial ecology to large-scale climate. Yet, routine measurments are unable to capture rapidly shifting near-surface soil heat and water processes involved in soil-water evaporation. The objective of this study was to determine the depth and location of the evaporation zone within soil. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, and temperature below a bare soil surface in Central Iowa during natural wetting/drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance, attributed to latent heat from water vaporization, provides an estimate of in situ soil-water evaporation. As the soil dried following rainfall, results show divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicates the location of a heat sink.associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provide depth and time patterns consistent with observed soil-water depletion patterns. Immediately after rainfall, evaporation occurred near the soil surface. Within six days after rainfall, the evaporation zone proceeded > 13 mm into the soil profile. Evaporation rates at the 3-mm depth reached peak values > 0.25 mm/h. Evaporation occurred simultaneously at multiple measured depth increments, but with time lag between peak evaporation rates for depths deeper below the soil surface. Implementation of fine-scale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation.